1. Field of Invention
This invention pertains to Positron Emission Tomography (PET) scanners having panel detectors. More particularly, this invention pertains to an apparatus and method for obtaining an estimate of a normalization array to correct for count rate effects associated with such a scanner.
2. Description of the Related Art
The application for patent, Ser. No. 10/103,276, filed Mar. 21, 2002, entitled “Normalization Apparatus for PET and SPECT Scanners and Method for Using Same”, has at least one common inventor with the present invention. The '276 application is incorporated herein by reference. In that application, it is discussed that various techniques are used for medical imaging. With respect to Positron Emission Tomography (PET), it is recognized that such is popular in radiology because of its ability to non-invasively study physiological processes and structures within the body.
PET is designed to measure the amount of radioactivity along many lines of response (LORs) that pass through the patient and are intercepted by the scanner's detectors. Measurement errors are always present, and in many cases must be corrected by the software that processes the measurements. In particular, the response measured on each LOR is subject to an error in magnitude. Normalization coefficients represent the relationship between the measured and actual magnitude of radiation and are used to correct the magnitude errors. Normalization coefficients are determined by measuring the difference in sensitivity or efficiency of the detectors in the scanners. Normalization of scanner data is usually performed by estimating the sensitivity or efficiency of a LOR.
As discussed by D. L. Bailey et al., “Quantitative Procedures in 3D PET”, The Theory and Practice of 3D PET, B. Bendriem and D. W. Townsend, Eds., Dordrecht, Netherlands: Kluwer Academic Publishers, 55-109 (1998), the non-uniformity of the different LORs in the sinogram must be corrected with an appropriate normalization in order to reduce artifacts created in a reconstructed image. The current techniques are based on: direct measurements of the normalization array; component-based methods simply using scatter-free 68Ge rods; and component-based methods using scatter-free 68Ge rods and methods to calculate or estimate and then subtract the scatter component. However, none of these methods are useful with PET scanners due to the high sensitivity of large area panel detectors.
In direct measurement methods, the elements of a normalization array are proportional to the reciprocal of counts acquired with a uniform source. This technique requires long acquisition time for good statistics in each element of the array. See D. W. Townsend, et al., “Three dimensional reconstruction of PET data from a multi-ring camera”, IEEE Trans. Nucl. Sci., vol. 36, no. 1, 1056-1065, (1989). In component-based methods, the normalization array is expressed as a product of factor functions, each being reduced to a much smaller number of parameters, as discussed by M. E. Casey, H. Gadagkar and D. Newport, “A component based method for normalization in volume PET”, Proc. 3rd Int. Meeting on Fully Three-Dimentional Image Reconstruction in Radiology and Nuclear Medicine, Aix-les-Bains, France, 67-71, (1995).
The lack of usefulness of these conventional methods with PET scanners having panel detectors is due to several factors. First, the PET scanner defines a relatively large field of view (FOV) and has high resolution, which makes it impractical to perform direct measurements of a normalization array. For example, a typical array is comprised of 320×256×3935 elements. The PET scanner further defines a relatively long FOV. Combined with a large tunnel and the absence of septa, the system is sensitive to scatter. Scatter-free sources cannot be used alone. Further, estimating and subtracting the scatter component is not the most effective method. Finally, none of the techniques described corrects for count rate effects, which are relevant for high sensitivity flat panel detectors.
It has been observed that normalization is count rate dependent. As a result, the normalization should be obtained in count rate conditions similar to the clinical scan on which it is applied in order to prevent ring artifacts in the transaxial views and banding in the axial direction.